One of the major causes of back pain and disability is degeneration of lumbar intervertebral discs. This source of pain may affect up to 80% of Americans over the course of their lifetimes. Furthermore, the economic impact of treating persons who suffer from disc degeneration is over $20 billion per year. Thus, disc degeneration has a significant effect on a sizeable portion of the population.
Disc degeneration may begin in childhood as cells within an intervertebral disc that produce the substrates that maintain disc hydration undergo normal senescence. At approximately the time that a person is four years old, his or her notochord cells are lost permanently, and their function is replaced by cells called chondrocytes. Both chondrocytes and notochord cells produce proteoglycans; however, chondrocytes also produce significant amounts of collagen, thereby causing discs to become less hydrated and firmer. This physiologic event marks the onset of disc degeneration.
Disc degeneration may be accelerated by several factors, including but not limited to injury and aging. Ideally, discs would regenerate on their own. Unfortunately, the intervertebral discs of the spine often fail to heal from injury and aging because of a paucity of blood vessels. The lack of a robust blood supply causes the intervertebral discs to obtain nutrition and to eliminate waste through the process of imbibition, which is the displacement of one fluid by another fluid that is immiscible. Over time, this process causes a lower pH to develop in the intervertebral discs and homeostasis via imbibition cannot be sustained.
When there is disc degeneration, patients can experience significant pain, and researchers and clinicians have long sought methodologies for treating this pain. One of the common historical approaches has been spinal fusion, which is the joining of two or more vertebrae. Spinal fusion addresses the degenerating disc by excision of the disc and subsequently, either fusing the spine with bone products or replacing the excised disc with a mechanical device.
Unfortunately, spinal fusions can cost between $60,000 and $100,000 per patient and can be associated with complications at twice the rate of complications that accompany nerve decompression surgery, which is another method for treating pain. Additionally, the result of spinal fusion surgery is a shift of mechanical stress to the intervertebral disc level above and/or below the fused or replaced disc. This common phenomenon is known as adjacent level disease/degeneration, and it too can contribute to discomfort in a patient. A further challenge for treating compromised discs by the common methods used by practitioners is that these methods are primarily non-biologic in nature, and thus face challenges in being accepted by the recipient.
Because of the high costs that are associated with lumbar fusions along with inconsistent outcomes and higher complication rates, researchers and clinicians have developed biologic and other non-fusion approaches for treating disc degeneration. Current non-fusion approaches for treating a degenerating disc include stabilizing the disc, rehydrating the disc, rebalancing the pH in the disc, intradiscal injections, provision of extracellular matrix proteins, intradiscal pressure reduction, disc denervation, disc nucleus replacement, stimulation of disc chondrocyte proliferation, non-ablation laser treatment and stem cell therapy. However, none of these strategies as currently employed provide optimally effective and efficient means for treating degenerated discs. Thus, there is a need to develop new means by which to address disc degeneration.